Need more details on the car and what you're trying to do with it. If it's a 86 regal with a n/a 3.8 or turd 305 v8, keep the gears and converter. If it's a turbo v6 the 4.10's are probably not ideal but not for the reasons you're stating. A Th400 with 4.10's will put you around 4k rpm's at...